The present invention relates to a mobile radio communication system such as a broad-zone automobile radio telephone system or the like, and more particularly, to a mobile radio communication control system capable of preventing base stations and/or mobile units from erroneously receiving control signals of other zones caused by interference with electromagnetic waves from the other zones.
Heretofore, in the above-referred type of mobile radio communication systems such as the one proposed in U.S. Pat. Nos. 3,898,390 or 4,028,500, a plurality of small zones respectively assigned to a plurality of base stations are combined to form a control zone, and a plurality of control zones jointly form a service area of a mobile radio communication system. To the plurality of small zones in each control zone are assigned different radio frequencies (message channels), and in these different control zones, the same radio frequencies are assigned to mutually corresponding small zones. Furthermore, it is a common practice that the mobile units in the service area are generally controlled by means of a common control channel from a stand-by state up to commencement of conversation, and control signals for switching message channels of the mobile units or for clearing a communication are transmitted and received by means of a message channel.
In the above-described zone system, mobile units which are talking through the same message channels can exist in a small zone (for instance, small zone 3A.sub.1, as will be described later) within a certain control zone (for instance, control zone 2A as will be described later) as well as in a small zone (for instance, small zone 3B.sub.1, as will be described later) within another control zone (for instance, control zone 2B as will be described later) corresponding to the small zone 3A.sub.1. Therefore, if a clearing control signal transmitted from a mobile unit which has terminated conversation in the small zone 3A.sub.1 should be erroneously received by a base station (for instance, base station 4B.sub.1) of the other small zone 3B.sub.1 through the same message channel, then a control station (for instance, control station 5B) controlling this base station 4B.sub.1 would perform a clearing operation in response to the termination of conversation at the mobile unit in the small zone 3B.sub.1. Accordingly, there is a disadvantage that the mobile unit in the small zone 3B.sub.1 is erroneously disconnected from communication.
Conversely, if a `disconnecting` control signal transmitted from the base station of the small zone 3A.sub.1 to a mobile unit in the same small zone through a message channel should be erroneously received by another mobile unit in another small zone 3B.sub.1 that is talking through the same message channel, the communication at the mobile unit in the small zone 3B.sub.1 is disconnected because the mobile unit in the small zone 3B.sub.1 performs disconnection processing as a clearing operation due to earlier hang-up by the general subscriber.
In addition, upon channel switching with communication also, a similar problem would occur. More particularly, in case where for a mobile unit in the small zone 3A.sub.1 is designated another message channel for a different small zone (for instance small zone 3A.sub.2) within the control zone 2A by means of a channel through which the mobile unit has been talking, a mobile unit in the small zone 3B.sub.1 may erroneously receive this control signal for designating a message channel and thus may switch its channel to the message channel for the small zone 3A.sub.2, so that normal communication would become impossible.
In order to prevent such electromagnetic wave interferences from another control zone, heretofore, it has been the practice to arrange the small zones 3A.sub.1 and 3B.sub.1 employing the same message channels sufficiently far apart from each other. However, in order the enhance the reliability of communication, it is necessary for this approach to increase the number of small zones within a control zone. In other words, the number of message channels must be increased to accommodate the increase of the small zones.
On the contrary, assuming that the total number of the assigned message channels is fixed, then the number of message channels for each small zone is decreased, reducing the number of mobile units which can be simultaneously serviced in each small zone.